Liquid crystal writing slate with improved light-transmission retention

ABSTRACT

A liquid crystal display device includes a base supporting a laminate which has an LC-layer (liquid crystal layer) formed of polymeric material holding micro-volumes of liquid crystal material, and a conductive layer on one surface of the LC-layer, the other surface of the LC-layer being substantially conductor-free, and apparatus for transitory application of potential through the LC-layer to the conductive layer to produce an image. The LC-layer has at least about 50% by weight of the polymeric material and less than 50% of the liquid crystal material by weight of the LC-layer, and the duration of image retention after transitory application of potential ceases is significantly extended.

RELATED APPLICATIONS

Related patent applications which are commonly-owned and filed concurrently herewith are as follows: U.S. Ser. No. 566,105, entitled "IMPROVED WRITING SLATE," an application of Frederick E. Nobile, John F. Harris III, Gary S. Silverman and Richard A. Baumann; U.S. Ser. No. 566,104, entitled "IMPROVED LIQUID CRYSTAL DISPLAY DEVICE," an application of Frederick E. Nobile, John F. Harris III, Gary S. Silverman and Richard A. Baumann; and U.S. Ser. No. 566,102, entitled "IMPROVED LIQUID CRYSTAL DISPLAY WITH DC IMAGING SYSTEM," an application of Frederick E. Nobile, John F. Harris III and Gary S. Silverman.

FIELD OF THE INVENTION

This invention is related generally to liquid crystal devices for light modulation and, more particularly, to liquid crystal devices of the type having a polymer holding micro-volumes of liquid crystal.

BACKGROUND OF THE INVENTION

Liquid crystal devices are used for display purposes and for a variety of other purposes utilizing the ability of liquid crystals to switch between two light-modulating characteristics. When an electrical potential is applied to a liquid crystal layer, the liquid crystals of such layer are polarized such that the layer becomes, for example, light-transmissive.

Retention of light-tranmissivity may be described as the time over which an LC-layer (liquid crystal layer) can pass light therethrough after cessation of the application of electrical potential or other excitation. Such retention is typically of very short duration in LC-layers, including those of the type formed of polymeric material holding micro-volumes of liquid crystal material. Indeed, quick reorientation to the non-field condition if often desirable.

A number of patents disclose information concerning retention of polarization. These include:

U.S Pat. No. 4,221,471 (Gurtler), which relates to a liquid crystal memory device and states that memory-stored information (and presumably the related image) is erased merely by removing the writing means, indicating that there is no retention of light-transmissiveness;

U.S. Pat. No. 4,693,557 (Fergason), involving a projector employing short-duration images said to last for several seconds;

U.S. Pat. No. 4,814,760 (Johnston et al.), involving a device said to need refreshing at a high frequency in order to permit the image to be seen without interruption;

U.S. Pat. No. 4,433,900 (Sekimura), which presents something of a different situation in that once formed, by extremely long applications of voltage, polarization is fixed; and

U.S. Pat. No. 4,671,618 (Wu et al.), involving a so-called polymer-dispersed liquid crystal material (a form of polymeric material holding micro-volumes of liquid crystal, as in this invention) said to have a memory time on the order of a few seconds or longer, such limited times being referred to as "extended memory."

The Wu et al. patent indicates that to achieve such "extended times" the LC-layer should contain at least 50% liquid crystal material.

It is apparent that it is desirable in certain liquid crystal applications to have polarization retention in a switchable system for a longer period than contemplated in such patents. For a writing/drawing slate or the like, a period of a few seconds is often inadequate.

The prior art includes certain liquid crystal devices which might be referred to as slates--devices addressed or imaged using stylus-like devices. Some examples of this are disclosed in U.S. Pat. No. 3,781,085 (Liebowitz), U.S. Pat. No. 4,525,032 (Hilsum), U.S. Pat. No. 4,723,836 (Kono et al.), and U.S. Pat. No. 3,886,311 (Rodgers et al.).

Such devices have certain disadvantages, particularly the relatively short duration of the image following cessation of the application of electrical potential. This disadvantage heretofore has made liquid crystal devices generally unsuitable as writing/display slates and for certain other applications.

The teachings in the Wu et al. patent about duration of polarization not only fail to provide what is needed for effective writing/drawing slates and the like, but the Wu et al. teaching of the use of large amounts of liquid crystal in the polymer-dispersed liquid crystal material tends to make any resulting product expensive because of the high cost of liquid crystal material.

A liquid crystal device having extended duration of the light-modulating characteristic induced by application of potential to an LC-layer would be a significant advance in the art.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an improved light-modulating liquid crystal device which overcomes some of the problems and shortcomings of the prior art.

Another object of this invention is to provide an liquid crystal device with an extended duration of a field-induced light-modulation characteristic after cessation of the application of electrical potential.

Another object of this invention is to provide a liquid crystal device with an extended light-modulation characteristic without the need for periodic refreshment.

Still another object of this invention is to provide a liquid crystal device with extended light-modulation characteristic without excessive costs from increased usage of liquid crystal in the LC-layer.

Yet another object of this invention is to provide an liquid crystal device having a laminate with improved charge-retaining capabilities.

Yet another object of this invention is to provide an liquid crystal device having a laminate with improved image duration suitable for writing/drawing slates and other similar applications.

Yet another object of this invention is to provide an improved liquid crystal device with a variety of means for transitory application of electrical potential through the LC-layer.

These and other important objects will be apparent from the description of this invention which follows.

SUMMARY OF THE INVENTION

This invention is an improvement in devices of the type having liquid crystal layers which provide two light-modulating characteristics. The invention overcomes certain problems and shortcomings of the prior art, including those mentioned above.

One way in which the invention overcomes such shortcomings relates to the concentration of liquid crystal material used in an LC-layer of the type having a polymeric material and micro-volumes of liquid crystal, and to the nature of the polymeric used to hold such material. The invention uses what may be term a high-resistance polymer and less than 50% liquid crystal material by weight of the LC-layer.

The invention includes: a laminate, preferably a flexible laminate, having an LC-layer (liquid crystal layer) with first and second surfaces (for example, bottom and top), the LC-layer formed of polymeric material which holds micro-volumes of liquid crystal material, the polymeric material and liquid crystal material being present in certain ratios, and a conductive layer on the first surface, the second surface being substantially conductor-free; and means for selective transitory application of electrical potential through the LC-layer to the conductive layer. More specifically, the LC-layer is at least 50% polymeric material by weight and less than 50% liquid crystal material. Furthermore, the polymeric material is chosen to have a resistivity of at least 10₁₅ ohm-centimeters.

With this arrangement and these proportions and materials, the laminate has charge-retaining capabilities such that a light-modulating characteristic formed by transitory application of potential persists for an significantly extended duration after such application ends. Transitory application of potential by the application means creates an image for display which remains after the cessation of potential application. Image retention can be for minutes or hours depending on a number of factors.

In preferred embodiments the LC-layer is of a material known as a polymer-dispersed liquid crystal. In highly preferred embodiments of this type, the laminate made having an LC-layer of polymer-dispersed liquid crystal material is very thin and flexible.

In highly preferred embodiments of the invention the LC-layer has at least about 60% of the polymeric material and less than about 40% of the liquid crystal material by weight of the LC-layer. In such cases, the thickness of the LC-layer may be increased slightly to provide enough light-reflectance or opacity in the laminate so that the contrast of images is sufficient.

In the most preferred embodiments the polymeric material of the LC-layer is chosen from polyvinyl acetates, acrylics and urethanes, and other similar polymers. Acrylic resins are most preferred.

The liquid crystal material is preferably a nematic liquid crystal substance selected from cyanobiphenyls, cyanoterphenyls, cyanophenylcyclohexanes, cyclohexyphenyl pyrimidines, phenylpyrimidines, alkylbenzoates, and other similar materials, and mixtures thereof.

Preferred embodiments of this invention include in the laminate a non-conductive overlayer on the second surface of the LC-layer to help protect the LC-layer from scratching and abrasion. A preferred overlayer material is polyester.

In certain preferred embodiments of this invention the conductive layer on the first surface of the LC-layer is a metal sheet. In certain other embodiments, the conductive layer may be transparent. In the latter case the laminate transmits light therethrough when the LC-layer is in one of the two visual characteristics.

In highly preferred embodiments, the means for application of potential includes a voltage source, most preferably a battery, a power supply electrically attached to such source for providing the electrical potential, and means for extending the potential from the power supply to the second surface.

The potential-extending means preferably may include an applicator having a first conductive region for contact with the laminate and a flexible electrical cord connecting the applicator to the power supply. Alternatively or in addition the extending means may include a separate exposed surface which is electrically connected to the power supply such that a person's body contacting such exposed surface can itself serve to extend the potential from the power supply to the second surface.

Further details of the invention are set forth below in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an edge elevation of a preferred display device in accordance with this invention, with portions enlarged and including a schematic diagram to show details.

FIG. 2 is an enlarged fragmentary edge elevation of a variation of the device of FIG. 1.

FIG. 3 is an electrical schematic diagram of one preferred form of electrical circuitry used in the device of this invention.

FIG. 4 is a simplified electrical schematic diagram of an alternative to the circuitry of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The figures show an improved liquid crystal display device 10--a writing/drawing slate in accordance with this invention. In the figures, like parts are identified by like numerals.

As illustrated, display device or slate 10 includes a base member 12 forming a flat platform 14 on which a laminate 16 is supported. Laminate 16 is secured to platform 14 by a thin adhesive layer (not shown).

Laminate 16 has an LC-layer 18 (that is, a layer including liquid crystal material) with a bottom or first surface 20 and a top or second surface 22. Laminate 16 also has a conductive layer 24 in intimate contact with first surface 20 of LC-layer 18, and a transparent non-conductive protective overlayer 26 in contact with second surface 22. Second surface 22 of LC-layer 18 is conductor-free. Laminate 16 has an image-displaying surface area 28.

LC-layer 18 is "switched" to provide either one of two light-modulating (that is, visual) characteristics. In one visual characteristic, LC-layer 18 is essentially transparent (light-transmissive) while in the other it is substantially opaque.

Images made by the contrast of the two different light-modulating characteristics of portions of LC-layer 18 appear at surface area 28. Before transitory application of electrical potential through a selected portion of LC-layer 18, the liquid crystal material at that portion of LC-layer 18 has a light-modulating characteristic (such as substantial opacity) limiting light transmissivity--by scattering and/or reflecting light. The transitory application of electrical potential through a selected portion of LC-layer 18 to underlying conductive layer 24 orients the liquid crystals at that portion of LC-layer 18 in one way, for example, such that the selected portion of LC-layer 18 becomes light-transmissive, a condition which persists until the liquid crystals of that portion of LC-layer 18 again assume orientations restoring the previous light-modulating characteristic.

LC-layer 18 is formed of a polymeric material holding micro-volumes of liquid crystal material. LC-layer 18 may be in various forms and made in various ways. The micro-volumes of liquid crystal may be wholly separated from one another by the polymeric material of the LC-layer, or the micro-volumes (or some of them) may be interconnected to some extent. Particular processing methods for production of LC-layer 18 itself are not within the scope of this invention.

In preferred embodiments, LC-layer 18 is a material of the type known as a "polymer-dispersed liquid crystal." A so-called polymer-dispersed liquid crystal material is made from a solution which includes a polymeric material and a liquid crystal material. Micro-volumes of the liquid crystal material form in the polymeric material as the liquid crystal material emerges from solution during a solvent-evaporation and polymer-curing (cross-linking) process.

LC-layer 18 is preferably made by formation on conductive layer 24. In forming LC-layer 18 of a polymer-dispersed liquid crystal material, as preferred, a solution as mentioned above is flowed onto conductive layer 24 into a liquid coating of substantially even thickness. The coating may then be treated, for example, with heat and time, to evaporate the solvent and cure the polymer. This process causes the formation (by phase separation) therein of micro-volumes of the liquid crystal material. The use of solvents is not always necessary; in some cases liquid crystal may be solubilized in the polymeric material itself at high temperatures and then form micro-volumes in the polymeric material during a cooling process.

For the LC-layer, one alternative to the so-called polymer-dispersed liquid crystal materials is a liquid crystal and polymer material made from an emulsion subjected to a drying process. The emulsion includes a water-soluble polymer and a liquid crystal material, and forms what is sometimes referred to as a liquid crystal encapsulation. Like the preferred polymer-dispersed liquid crystal materials, such liquid crystal encapsulations are polymers holding micro-volumes of liquid crystal.

Whatever formation process is used, the polymeric and liquid crystal materials used in forming LC-layer 18 are selected to provide extended image retention after cessation of transitory application of electrical potential through LC-layer 18 to conductive layer 24.

In keeping with this objective, in this invention the polymeric material of LC-layer 18 is of the type having a high electrical resistivity. That is, the polymeric material has a resistivity of at least about 10¹⁵ ohm-centimeters. With such a polymeric material, and an appropriate choice of liquid crystal material, the image on the image-display surface is retained for a substantial period after cessation of the transitory application of electrical potential through LC-layer 18.

The polymeric material is preferably selected from the polyvinyl acetates, acrylics and urethanes, and other polymeric materials having similar light-transmissive and high electrical resistivity. The most highly preferred polymeric materials are acrylic resins.

The liquid crystal material is preferably a nematic liquid crystal substance selected from cyanobiphenyls, cyanoterphenyls, cyanophenylcyclohexanes, cyclohexyphenyl pyrimidines, phenylpyrimidines, alkylbenzoates, other liquid crystal materials having similar properties, and mixtures of such materials. Suitable liquid crystal materials are commercially available from Hoffmann LaRoche (such as ROTN 132, 520, 711 and 3520) and E. Merck Chemical Company (such as E-7, E-201 and ZLI-1253), each of such liquid crystal materials being mixtures.

In accordance with this invention, LC-layer 18 includes at least 50% of the polymeric material and less than 50% of the liquid crystal material by weight of LC-layer 18. It has been found that decreasing the amount of liquid crystal material relative to polymeric material in the LC-layer actually yields images which last longer after transitory application of electrical potential ceases. In certain preferred embodiments, LC-layer 18 is made up of at least about 60% of the polymeric material and less than about 40% of the liquid crystal material by weight.

Image retention is significantly enhanced by using an LC-layer formed with more than about 50% polymeric material. If less is used, image retention is impaired. Image retention is further enhanced if the LC-layer has in excess of 60% polymeric material. However, it should be noted that when the amount of liquid crystal material relative to polymeric material is significantly decreased, it may be advisable for LC-layer 18 to be somewhat thicker to increase the opacity of LC-layer 18.

One preferred material for LC-layer 18 is made by combining two parts by weight of a 54% solids solution (in toluene) of Joncryl™ 587; (a commercially-available hydroxyl-functional acrylic resin made by S. C. Johnson and Son, Inc., Racine, Wisconsin) with one part by weight of Hoffmann LaRoche ROTN-132 liquid crystal mix, to form a homogeneous solution. This solution is coated on conductive layer 24 and solvent evaporation forms a white LC-layer usable in this invention, ranging from somewhat translucent to substantially opaque depending on thickness. 52% of the resulting LC-layer is of an acrylic polymer, with 48% being liquid crystal material.

Another material for LC-layer 18 is made by forming a homogeneous solution using four parts by weight of a 40% solids solution (in ethyl acetate and toluene) of a hydroxy-functional polyvinyl acetate (such as ASB 516 from Air Products and Chemicals, Inc.) with one part of Hoffmann LaRoche ROTN-570 liquidcrystal mix. This solution was applied to a conductive layer (such as the metallized side of a metallized polyester film). In a solvent-evaporation/polymer-curing process a tack-free white layer was formed, ranging from somewhat translucent to substantially opaque. The resulting LC-layer has 62% polymer and 38% liquid crystal material.

Those areas of the LC-layers described above through which a transitory electrical potential was applied become transparent upon and after such application, by virtue of molecular orientation of the liquid crystals. With appropriate application of potential (e.g., about 900 volts at about 9 microamperes), the images created by such liquid crystal transparency persist for several minutes.

A great many other variations in LC-layer formulation and production method are possible With such variations, and/or with variations on voltage application, the image can persist for many hours, in some cases even for more than one full day.

A great many other variations in LC-layer formulation and production method are possible. Some further examples, each involving so-called polymer-dispersed liquid crystal materials, are given below. With such formulations, the LC-layers are made and used in the manner described above.

In one example, a solution is formed from: 1.0 parts by weight of a 50% solids solution (in an ethyl acetate based solvent) of a Rohm & Haas reactive acrylic resin known as AU-1033; 0.5 parts of E-201, a liquid crystal mixture from Merck Chemical Company; 0.75 parts of a 10% solids solution (in a toluene, ethyl acetate solvent) of isocyanate cross-link composition N75 from Mobay Chemicals; 0.09 parts of a 0.1% solution (in ethyl acetate) of Kenrich Petrochemicals Inc. NZ39, adhesion promoter; and 0.136 parts of a 0.1% solution (in ethyl acetate) of dibutyl tin dilaurate (a catalyst). An image appropriately formed in an LC-layer made with this formulation is visible for several hours following transitory application of electrical potential.

In another example, a solution is formed from: 1.6 parts by weight of a 38% solids solution of POI-301, an isocyanate-reactive acrylic resin available from Peregrine Optifilm, Inc., DePere, Wisconsin; 0.6 parts of ROTN-570, a liquid crystal mixture from Hoffmann LaRoche; 0.12 parts of an 80% solids solution in N-butyl acetate) of Desmophen 670A-80, a polyester poyol from Mobay Chemicals; and 0.136 parts of a 0.1% solution (in ethyl acetate) of dibutyl tin made with this formulation is visible for more than a day following transitory application of electrical potential.

Conductive layer 24 may be in a variety of forms and made of a variety of materials. In one preferred form, conductive layer 24 is a thin metal sheet (foil) or a relatively rigid metal plate. This is illustrated by FIG. 2. Conductive layer 24 may also be a non-conductive sheet with a conductive surface material 30 thereon, such as a metallized surface or other conductive coating applied thereto. See FIG. 1, illustrating one such highly preferred form. When using such materials, the metallized or other conductive surface material is preferably in contact with first surface 20 of LC-layer 18. In one highly preferred example, conductive layer 24 is made of a non-conductive polyester film which is metallized on one surface. Such metallized polyester film may have a thickness on the order of 0.180 mm.

Conductive coatings also include some available in paint and pigment form. One form of conductive coating is a conductive ink, which allows ready printing for a variety of image-making purpose.

Conductive layer 4 may be made using a transparent non-conductive sheet to which a transparent conductive material, such as indium tin oxide (ITO), a transparent ink or other substance, or very thin coating of a metal (such as aluminum or gold), is applied. The resulting conductive layer is transparent, which allows all layers of laminate 16 (conductive layer 24 LC-layer 18, and overlayer 26) to be transparent. A variety of interesting uses may be made of such transparent laminate.

While it is preferred that a conductive surface of the conductive layer being contact with first surface 20 of LC layer 18, in some cases a non-conductive surface of the conductive layer may being contact with first surface 20, and/or an additional layer which is non-conductive may be between conductive layer 24 and first surface 20. In such cases, applied voltage may need to be at somewhat higher levels.

In preferred embodiments, laminate 16 includes not just conductive layer 24 and LC-layer 18, but overlayer 26. As previously mentioned, overlayer 26 serves to protect LC-layer 18 from physical damage which can occur during careless writing and drawing, or from other abuse. Overlayer 26, which is on intimate contact with second surface 22 of LC-layer 18, is non-conductive so that second surface 22 is substantially conductor-free.

Preferred non-conductive overlayers are transparent and have a thickness on the order of 0.0002-0.002 inch (0.0051-0.051 mm), with 0.0005 inch (0.0128 mm) being highly preferred. A highly preferred material for overlayer 26 is polyester.

Overlayer 26, as previously noted, may be secured to second surface 22 by an adhesive, such as a polyurethane adhesive. In some cases, however, an adhesive is not required; sufficient retention of overlayer 26 on second surface 22 is accomplished by thermal bonding between the layers.

Regardless of whether laminate 16 has an overlayer, second surface 22 of LC-layer 18 is substantially conductor-free. Image-displaying surface area 28 may have identifiers such as numerals, graph lines, axis markers or the like formed of conductive material. In some cases minimal conductive materials may be on the overlayer at a position away from contact with second surface 22. However, LC-layer 18, on what can be referred to as its image-accepting region (that is, the area of LC-layer 18 to which a stylus or other applicator, as described, applies by its motions variable images of any sort), is free of a fixed conductor on second surface 22 which would serve an image-applying function with regard to such variable images. The term "conductor-free" refers to the fact that the liquid crystal display at last along the aforesaid image-accepting region is not of a type including a standard sandwich of liquid crystal material between conductive layers.

It has been found that overlayer 26, or more accurately an appropriate choice of material for overlayer 26, not only protects LC-layer 18, but such overlayer also significantly improves the time of image retention (after cessation of transitory application of potential) over the retention times in embodiments not including such overlayer. The precise reason for improved image retention times is not known; however, overlayer 26, in conjuction with the other layers, may act as an improved multi-layer capacitor after transitory application of potential ends.

The devices of this invention have image retention times which are significantly extended when compared to the teachings of the prior art. Durations which are achieved in the devices of this invention are more than adequate for a number of information display devices, including children's slates, blackboard-like lecture-room displays, and a variety of other devices.

In preferred embodiments, laminate 16 includes means for inhibiting (blocking) the passage of ultraviolet light. The inhibiting means is applied to LC-layer 18 and serves to extend substantially the useful life of display device 10 despite substantial exposure of laminate 16 to sunlight and/or other ultraviolet light. Permanent destructive effects of the light-modulating functions of LC-layer 18 are avoided and product life extended.

The ultraviolet-inhibiting means may be applied to LC-layer 18 in various ways. In one preferred embodiment, an ultraviolet-inhibiting substance was applied in making overlayer 26 such that it was substantially homogeneous therein. Incorporation of the inhibiting means into overlayer 26 may be before overlayer 26 is applied to second surface 22 of LC-layer 18. One preferred material for overlayer 26 is a polyester material known as "UV Absorber Film," available from Transilwrap, of Chicago, Illinois, which has an ultraviolet-inhibiting composition incorporated therein. (one alternative is an separate layer forming a portion of overlayer 26. Another alternative is to incorporate an ultraviolet-inhibiting composition in an adhesive layer.

As shown in the figures, slate 10 has means secured to base member 12 for selective transitory application of electrical potential through LC-layer 18 to conductive layer 24 to produce an image appearing at image-displaying surface area 28. Slate 10 also has means secured to base 12 to facilitate erasing of the image.

As shown in FIGS. 1 and 3, the means for transitory application of electrical potential includes a battery 32, to serve as voltage source, a power supply 34 electrically attached to battery 32, and means for extending the potential from power supply 34 to second surface 22 of LC-layer 18.

One preferred extending means includes a conductive exposed contact surface 36 which is electrically connected to power supply 34. A user who contacts exposed surface 36, such as with a hand or finger, can extend the potential from power supply 34 to second surface 22 of slate 10 by drawing or writing on laminate 16 with a finger of his other hand.

Another preferred means for extending the potential from power supply 34 to second surface 22 includes an applicator 38. Applicator 38 has a first conductive region 40 for contact with laminate 16 and a flexible electrical cord 42 which electrically connects applicator 38 to power supply 34.

In one highly preferred embodiment shown in the drawings, applicator 38 is an elongate implement with a distal end 44 and a proximal end 46. First conductive region 40 forms and extends from distal end 44 toward proximal end 46. A non-conductive hand-hold region 48 is positioned adjacent to first conductive region 40. Electrical cord 42 is attached to applicator implement 38 at proximal end 46. Applicator implement 38 is a stylus for writing and drawing on image-displaying surface 28 of slate 10.

Applicator 38 also has a second conductive region 50 which is electrically joined to first conductive region 40 and positioned such that hand-hold region 48 extends between first and second conductive regions 40 and 50. Such regions are electrically connected together by a wire or solid bar extending therebetween inside hand-hold region 48. Instead of writing or drawing in the normal manner, second conductive region 50 can be held by a user in one hand to allow the user to write or draw with the fingers of his other hand. Second conductive region 50 is an alternative to exposed contact surface 36.

As shown in the figures, slate 10 includes means to facilitate erasing of images produced as described above. The erasing means, which is an electrical pathway for short-circuiting image-displaying surface 28 to conductive layer 24, includes an erase-contact 54 electrically joined to conductive layer 24 and positioned for engagement by the hand of a user so that the other hand, or a conductive member held in such other hand, can be applied to surface area 28 in an erasing motion. Thus, a person's body forms a portion of the electrical pathway which shorts surface 28 to conductive layer 24.

As explained above, images may be formed on the liquid crystal display of this invention by selective transitory application of DC electrical potential through LC-layer 18 to conductive layer 24. Since second surface 22 of laminate 16 is substantially conductor-free, the application of DC electrical potential forms images without degradation of the laminate, as occurs when DC is applied in a laminate including an indium tin oxide transparent conductive layer.

DC is highly preferred because its use avoids what might be called "zebra striped" images of non-uniform intensity. To put it differently, DC electrical potential produces images of uniform intensity independent of the application method, that&is, independent of whether such potential is applied to surface area 28 without surface-crossing motion, with slow surface-crossing strokes, or with very rapid strokes.

FIG. 3 illustrates the circuitry of DC power supply 34, as used in this invention. Power supply 34 is of the flyback type. It utilizes battery 32. Power supply 34 includes an optional switch 56, connected between terminals 58 and 60, which serves to turn power supply 34 on and off. The output voltage available at output terminal 62 is preferably determined and set to provide acceptable slate operation accommodating various factors, such as the thickness of LC-layer 18, the thickness of overlayer 26, the materials for the various layers, etc.

A preferred output voltage from a power supply is within the range of about 150-2000 volts DC. For a laminate without an overlayer, the lower portion of this range should suffice. With overlayer 26 in place, a preferred output voltage range is about 300-2000 volts DC. For polyester overlayers having a preferred thickness range of about 0.0002-0.002 inch (0.0051-0.051 mm), output voltage is preferably within the range of about 500-1200 volts DC.

Power supply 34 operates by rapidly switching MOSFET 64 to induce a voltage in both portions of an inductor 66. These voltages are rectified, filtered and directed to output terminal 62. A regulator network 68 maintains the output voltage within a relatively narrow range as selected and set using resistor 70.

Referring next to FIG. 4, a power supply 72 in the form of an AC-to-DC converter is shown. Such converter has

an AC input 74 (for example, 120V-60 Hz), and an output voltage as described above. The transformer 76 is preferably a high-permeability type auto-transformer to permit selection of the appropriate output voltage, which appears at terminals 78.

It should be appreciated that, in both FIG. 3 and FIG. 4, those points identified by the ground symbol are electrically connected directly or indirectly to one another and to conductive layer 24. It is also to be appreciated that a negative rather than a positive potential can be used for image creation. In yet another variation, a voltage (positive or negative) is applied to the conductive layer and a person's body, contacting the image-displaying surface area (through a finger or hand-held conductive implement) and no other portion of the device, functions as a around and creates an image. All of extending potential from a power supply to the second surface of the LC-layer.

In both of the illustrated power supplies, the output current is within the range of about 1-10 microamperes at the elevated output voltage. Furthermore, the power supplies are current-limited to a maximum output current of 10 microamperes.

Another voltage source is a piezoelectric generator which, for example, may be located in an applicator or elsewhere in the display device.

Referring now in greater detail to laminate 16, LC-layer 18 is preferably made by formation thereof on conductive layer 24. Conductive layer 24 may be unrolled onto a table to receive the liquid from which LC-layer 18 is formed. For an LC-layer 18 of polymer-dispersed liquid crystal material, as preferred, a solution including a polymeric material and a liquid crystal material is flowed onto conductive layer 2 into a coating of substantially even thickness. The coated conductor layer is then passed through an oven until the liquid solidifies by solvent evaporation and curing/cross-linking of the polymer. A preferred thickness for LC-layer 18 is about 0.00025 inch (0.00635 mm). Further curing (for example, for 24 hours) may be while the web, including conductive layer 24 and the formative LC-layer 18 thereon, is coiled into a roll.

Following curing, a very thin layer of adhesive (not shown) is applied to second surface 22 of LC-layer 18. Overlayer sheet 26 is then placed thereon and squeegeed. The resulting laminate is then die-cut to the desired size, suitable for placement on platform 14 and attachment thereto.

The device of this invention may be operated, as already indicated above, in the following manner:

After energizing power supply 34 by closing switch 56, an image may be made to appear at image-displaying surface area 28 by contacting such area with first conductive region 40 of applicator 38, in writing or drawing motions. Alternatively, images may also formed by a person's body, such as by contacting surface area 28 with the finger of one hand while the other hand is contacting exposed contact surface 36 or second conductive region 50 (or, for that matter, first conductive region 40). Images are formed by the liquid crystals in LC-layer 18 assuming a light-transmissive orientation, and such images last well after cessation of the transitory application of potential.

To erase images, the user touches erase-contact 54, typically with one hand, and simultaneously touches the surface area with a finger or fingers of the other hand, or with a conductor held in such hand. This shorts surface area 28 to conductive layer 24.

The device of this invention may also be used in the following manner to create and erase "reverse" images:

While contacting second conductive region 50 (or first conductive region 40 or exposed contact surface 36), a user can wipe across the entirety of surface area 28 using a hand or a hand-held conductive piece. This brings all of LC-layer 18 into a first light-transmissive visual characteristic. If the surface of conductive layer 24 which is in contact with first surface 20 of LC-layer 18 is reflective, as preferred for writing/drawing slates, the entire surface area will thereupon be mirror-like. In any event, conductive layer 24 is exposed. The user then contacts erase-contact 24 with one hand and simultaneously touches the displaying surface with a finger of the other hand (or a conductive implement held in such hand). This brings the contacted portions of image-displaying surface 28 to a second (opaque) visual characteristic. Thus, the image is the reverse of the images made in the manner previously described.

As used herein in describing writing/drawing and erasing on image-displaying surface 28, the term "contact" (in all its forms) means either to actually touch surface area 28 or to come in very close but non-touching proximity thereto. It is not necessary in many situations to actually touch such surface.

Referring again to the means for application of potential, an untethered applicator can be used as an alternative to applicator 38. An untethered applicator could be like applicator 38 in all respects, except that it could have a self-contained battery and power supply, and no electrical cord for attachment to the remainder of the device.

While the principles of this invention have been described in connection with specific embodiments, it should be understood clearly that these descriptions are made only by way of example and are not intended to limit the scope of the invention. 

We claim:
 1. In a device of the type having a liquid crystal layer providing two light-modulating characteristics, the improvement comprising:a laminate having an LC-layer with first and second surfaces and formed of polymeric material holding micro-volumes of liquid crystal material, and a conductive layer along the first surface, the second surface conductor-free on or along an image-accepting region of the laminate; movable means for selective transitory application of electrical potential through the LClayer to the conductive layer; and the LC-layer comprising at least 50% of the polymeric material and less than 50% of the liquid crystal material by weight of the LC-layer, the polymeric material having a volume resistivity of at least about 10¹⁵ ohm-centimeters,whereby the laminate has charge-retaining capabilities such that a light-modulating characteristic formed on the image-accepting region by transitory application of potential thereto persists for an extended duration following such application.
 2. The device of claim 1 wherein the LC-layer is a polymer-dispersed liquid crystal.
 3. The device of claim 2 wherein the laminate is flexible.
 4. The device of claim 1 wherein the LC-layer comprises at least about 60% of the polymeric material and less than about 40% of the liquid crystal material by weight of the LC-layer.
 5. The device of claim 1 wherein the laminate further includes a non-conductive overlayer on the second surface, whereby the LC-layer is protected.
 6. The device of claim 5 wherein the non-conductive overlayer is polyester.
 7. The device of claim 1 wherein the polymer is selected from the group consisting of polyvinyl acetates, acrylics and urethanes.
 8. The device of claim 7 wherein the polymer is an acrylic resin.
 9. The device of claim 8 wherein the laminate further includes a non-conductive overlayer on the second surface, whereby the LC-layer is protected.
 10. The device of claim 9 wherein the non-conductive overlayer is polyester.
 11. The device of claim 1 wherein the conductive layer is transparent, whereby the laminate transmits light therethrough in one of said two visual characteristics.
 12. The device of claim 1 wherein the means for application of potential comprises:a voltage source; power supply means electrically attached to the voltage source for providing said electrical potential; and means for extending the potential from the power supply to the second surface.
 13. The device of claim 12 wherein the extending means comprises:an applicator having a conductive region for contact with the laminate; and a flexible electrical cord connecting the applicator to the power supply.
 14. The device of claim 12 wherein the extending means comprises an exposed surface electrically connected to the power supply, whereby a person's body contacting the exposed surface can extend the potential from the power supply to the second surface. 